1. Field of the Invention
This invention relates to a drooping elimination device for a semiconductor laser in a laser beam recording apparatus for recording a continuous tone image on a photosensitive material by scanning the photosensitive material with a laser beam whose optical intensity is modulated analog-wise in accordance with an image signal.
2. Description of the Prior Art
Light beam scanning recording apparatuses wherein a light beam is deflected by a light deflector and scanned on a photosensitive material for recording an image on the photosensitive material have heretofore been used widely. A semiconductor laser is one of the means used for generating a light beam in the light beam scanning recording apparatuses. The semiconductor laser has various advantages over a gas laser or the like in that the semiconductor laser is small, cheap and consumes little power, and that the laser beam can be modulated directly by changing the drive current.
FIG. 2 is a graph showing the optical output characteristics of the semiconductor laser with respect to the drive current. With reference to FIG. 2, the optical output characteristics of the semiconductor laser with respect to the drive current change sharply between a LED region (natural light emission region) and a laser oscillation region. Therefore, it is not always possible to apply the semiconductor laser to recording of a continuous tone image. Specifically, in the case where intensity modulation is carried out by utilizing only the laser oscillation region in which the optical output characteristics of the semiconductor laser with respect to the drive current are linear, it is possible to obtain a dynamic range of the optical output of only approximately 2 orders of magnitude at the most. As is well known, with a dynamic range of this order, it is impossible to obtain a continuous tone image having a high quality.
Accordingly, as disclosed in, for example, Japanese Unexamined Patent Publication Nos. 56(1981)-115077 and 56(1981)-152372, an attempt has been made to obtain a continuous tone image by maintaining the optical output of the semiconductor laser constant, continuously turning on and off the semiconductor laser to form a pulsed scanning beam, and controlling the number or the width of pulses for each picture element to change the scanning light amount.
However, in the case where the pulse number modulation or the pulse width modulation as mentioned above is carried out, in order to obtain a density scale, i.e. a resolution of the scanning light amount, of 10 bits (approximately 3 orders of magnitude) when the picture element clock frequency is 1 MHz for example, the pulse frequency must be adjusted to be very high (at least 1 GHz). Though the semiconductor laser itself can be turned on and off at such a high frequency, a pulse counting circuit or the like for control of the pulse number or the pulse width cannot generally be operated at such a high frequency. As a result, it becomes necessary to decrease the picture element clock frequency to a value markedly lower than the aforesaid value. Therefore, the recording speed of the apparatus must be decreased markedly.
Also, with the aforesaid method, the heat value of the semiconductor laser chip varies depending on the number or the widths of the pulses which are output during the recording period of each picture element, so that the optical output characteristics of the semiconductor laser with respect to the drive current change, and the exposure amount per pulse fluctuates. As a result, the gradation of the recorded image deviates from the correct gradation, and a continuous tone image of a high quality cannot be obtained.
On the other hand, as disclosed in Japanese Unexamined Patent Publication No. 56(1981)-71374 for example, it has been proposed to record a high-gradation image by combining pulse number modulation or pulse width modulation with optical intensity modulation. However, also with the proposed method, the heat value of the semiconductor laser chip varies depending on the number or the widths of the pulses which are output during the recording period of each picture element, so that the exposure amount per pulse fluctuates.
In view of the above, in order to record a high-gradation image of a density scale of approximately 10 bits, i.e. approximately 1024 levels of gradation, it is desired that a dynamic range of the optical output be adjusted to approximately 3 orders of ten by carrying out optical intensity modulation over the LED region and the laser oscillation region as shown in FIG. 2. However, the optical output characteristics of the semiconductor laser with respect to the drive current are not linear over the two regions. Therefore, in order to control the image density at an equal density interval for a predetermined difference among the image signals so that a high-gradation image can be recorded easily and accurately, it is necessary to make linear the relationship between the light emission level instructing signal and the optical output of the semiconductor laser by compensation of the optical output characteristics of the semiconductor laser with respect to the drive current.
As a circuit for making linear the relationship between the light emission level instructing signal and the optical output of the semiconductor laser, it has heretofore been known to use an optical output stabilizing circuit (an automatic power control circuit, hereinafter abbreviated to the APC circuit) for detecting the optical intensity of a laser beam and feeding back a feedback signal, which corresponds to the detected optical intensity, to the light emission level instructing signal for the semiconductor laser. FIG. 3 is a block diagram showing an example of the APC circuit. The APC circuit will hereinbelow be described with reference to FIG. 3. A light emission level instructing signal Vref for instructing the optical intensity of a semiconductor laser 1 is fed to a voltage-to-current conversion amplifier 3 via an addition point 2. The amplifier 3 feeds a drive current proportional to the light emission level instructing signal Vref to the semiconductor laser 1. A laser beam 4 emitted forward by the semiconductor laser 1 is utilized for scanning a photosensitive material via a scanning optical system (not shown). On the other hand, the intensity of a laser beam 5 emitted rearward from the semiconductor laser 1 is detected by a pin photodiode 6 disposed for optical amount monitoring, for example, in a case housing the semiconductor laser 1. The intensity of the laser beam 5 thus detected is proportional to the intensity of the laser beam 4 actually utilized for image recording. The output current of the pin photodiode 6 which represents the intensity of the laser beam 5, i.e. the intensity of the laser beam 4, is converted into a feedback signal (voltage signal) Vpd by a current-to-voltage conversion amplifier 7, and the feedback signal Vpd is sent to the addition point 2. From the addition point 2, a deviation signal Ve representing a deviation between the light emission level instructing signal Vref and the feedback signal Vpd is output. The deviation signal Ve is converted into a current signal by the voltage-to-current amplifier 3 and is utilized for operating the semiconductor laser 1.
With the aforesaid APC circuit, the intensity of the laser beam 5 is proportional to the light emission level instructing signal Vref in the case where ideal linearity compensation is effected. Specifically, the intensity Pf of the laser beam 4 (i.e. the optical output of the semiconductor laser 1) utilized for image recording is proportional to the light emission level instructing signal Vref. However, it is not always possible to achieve such ideal linearity compensation. Therefore, as proposed in, for example, Japanese patent application No. 61(1986)248873, it is considered to provide a correction table for correcting the light emission level instructing signal so as to compensate for nonlinearity of the optical output characteristics of the semiconductor laser with respect to the drive current.
In the case where the light emission level instructing signal Vref is of a very low frequency that is close to a direct current, the relationship between the light emission level instructing signal and the optical intensity of the laser beam can be made linear by use of the aforesaid correction table.
However, in the case where the light emission level instructing signal Vref is of a high frequency, different problems further arise as described below. FIG. 4 is a graph showing dependence of the drive current-optical output characteristics of the semiconductor laser as shown in FIG. 2 on the temperature in the case housing of the semiconductor laser. As shown in FIG. 4, when the drive current is constant, the optical output of the semiconductor laser is lower as the temperature in the case housing of the semiconductor laser is higher. In general, in the case where the semiconductor laser is applied to a laser beam recording apparatus or the like, the apparatus is provided with a control means for maintaining the temperature in the case housing of the semiconductor laser at a predetermined value. However, it is impossible to restrict even transitional variations of the temperature of a laser diode chip arising when the drive current is applied to the semiconductor laser. FIGS. 5A, 5B, 5C, 5D and 5E are explanatory graphs showing the drooping characteristics of the semiconductor laser. Specifically, at the time the drive current is applied step-wise to the semiconductor laser as shown in FIG. 5A, the temperature of the laser diode chip changes transitionally as shown in FIG. 5B until it comes into the steady condition by the aforesaid control for maintaining the temperature in the case housing of the semiconductor laser at a predetermined value. As a result, in accordance with the characteristics as shown in FIG. 4, the optical output of the semiconductor laser changes as shown in FIG. 5C. Such changes in the optical output are referred to as the drooping characteristics of the semiconductor laser.
FIG. 5C shows the characteristics of the semiconductor laser itself. In the case where the optical output of the semiconductor laser is controlled by the APC circuit as shown in FIG. 3, the operation of the semiconductor laser is stabilized, but the effects of the drooping characteristics still remain uneliminated as shown in FIG. 5D. When the optical output of the semiconductor laser changes as shown in FIG. 5D, the problem that, for example, the density of the recorded image fluctuates with the passage of time with respect to the same image signal arises.
In the case where the loop gain of the APC circuit is adjusted to be very high, the effects of the drooping characteristics can be eliminated almost completely. However, in order to record the continuous tone image as mentioned above, the light emission response characteristics of the semiconductor laser must be substantially increased in the case where the semiconductor laser is to be operated by quickly changing the light emission level instructing signal Vref analog-wise. Actually, for this reason, the loop gain of the APC circuit cannot be adjusted to be so high.